Much discussion has arisen as to their mode of origin, but it is probable they were caused by the erosive action of ice, since glaciers occupied the glens where they occur and wore down the rocks along the sides and bottom; but it is a point of difficulty in this theory whether ice could have eroded the deepest of the hollows.
To the strike, and cutting through opposed escarpments by deep valleys, and those following the line of strike along a bed of easily eroded rock.
Some of the western valleys have also in part of their length beeIi converted into U-shaped troughs; the famous Yosemite Vailey, eroded in massive granite, with side cliffs 1000 or 2000 ft.
The outlet of this glacial lake, called river Warren, eroded a large channel in which the Minnesota river, of to-day is an evident misfit.
That those to the westward have long been inactive is shown by the destruction of craters by denudation, by deep ravines, valleys and tall cliffs eroded on the mountain sides, especially on the windward side, by the depth of soil formed from the disintegrated rocks, and by the amount as well as variety of vegetable life.
The hilly aspect of several parts of the central plateau is not due to foldings of the strata, which for the most part appear to be horizontal, but chiefly to the excavating action of the rivers, whose valleys are deeply eroded in the plateau, especially on its borders.
In area) filling the hollow of an old volcano, and with rivers which have eroded their beds in the tuffs to a depth of 300 ft.
The most striking feature of the embayment is the broad valley which the Mississippi has eroded across it.
Shales occur, and the passage of the streams through deep rifts or gorges across the hard limestone anticlinals, which stand in irregular series of parallel ridges with the eroded valleys between.
Deep, hardly wider at the top than at the bottom, in the heavy Triassic sandstones of southern Utah; but the most famous example is the Grand Canyon (qv.) of Arizona, eroded by the Colorado river across the uplifted platform of Carboniferous limestone.